In the past, magnetoresistive ("MR") heads and sensors have been used for reading magnetic information stored on both magnetic disk and tape storage systems. Magnetoresistive heads are capable of producing high signal output with low noise that is independent of media velocity if the flying height is a constant. This high signal output and low noise, i.e., high signal to noise ratio, makes possible media noise limited overall system performance at high areal storage densities.
Magnetoresistive heads detect magnetic transitions through the resistance change of an MR read element which varies as a function of the strength and direction of the magnetic flux impinging on the read element. By applying an electrical sense current to the MR head structure an output voltage may be generated which is proportional to the resistance of the material, and in turn proportional to the magnetic field from the media.
A known first class of MR heads typically utilize a single-domain thin film (on the order of 500 angstrom units in thickness) permalloy sensor element with a small internal crystalline anisotropy axis, or "easy axis," typically induced during film deposition by the application of a static magnetic field. These MR heads function according to the anisotropic magnetoresistive ("AMR") effect, where the resistance of the MR sensor element varies as a function of square of the cosine (cos.sup.2) of the angle between the magnetization direction of the sensor element and the sense current direction.
Biasing techniques are typically employed with AMR heads so that the MR sensor element operates in the linear region of its transfer curve and avoids the quadratic response of the cos.sup.2 function. Biasing is often achieved by the addition of a second permalloy layer or soft adjacent layer ("SAL") separated by a spacer from the MR sensor element. A more detailed description of MR heads that operate according to the AMR effect is found in "Magnetoresistive Head Technology", D. Markham et al., Proc. of the Elect. Chem. Soc. Vol. 90-8, p. 185 (1990), which is incorporated herein by reference.
Recently, a second class of MR heads, commonly referred to as spin valves, have been developed that have a more pronounced MR effect. Spin valve MR heads are multilayered structures that typically have two or more magnetic layers separated by a non-magnetic layer. As discussed in U.S. Pat. No. 5,287,238 to Baumgart et al., the physical phenomena associated with the change in resistance of such layered magnetic structures is variously referred to as the giant magnetoresistive ("GMR") effect or the "spin valve" effect. This effect has been attributed to the spin-dependent transmission of the conduction electrons between magnetic layers through a non-magnetic layer and the accompanying spin-dependent scattering at the layer interfaces.
Unlike conventional AMR heads, the resistance of a spin valve MR head does not change as a function of applied sense current direction. Rather, as described in "Giant Magnetoresistance in Soft Ferromagnetic Multilayers", Dieny et al., American Physical Society Vol. 43, No. 1 p. 1297 (1991), which is incorporated herein by reference in its entirety, the in-plane resistance between a pair of ferromagnetic layers separated by a non-magnetic layer varies as a function of the cosine of the angle between the magnetization in the two layers.
In spin valve MR heads one or more of the magnetic layers is typically "pinned" through the use of exchange coupling, as is well known in the art, such that their magnetization direction is fixed while the MR sensor element layer is free to rotate under the influence of the fringing fields from the magnetic transitions stored on the media. The fringing fields from the media causes the magnetization direction of the sensor element to rotate relative to the fixed magnetization direction of the pinned magnetic layer or layers. As is the case with conventional AMR heads, spin valve MR heads having an easy axis perpendicular to the magnetic direction of the field to be sensed are known.
U.S. Pat. No. 5,159,513 to Dieny et al., and U.S. Pat. No. 5,206,590 also to Dieny et al., describe a spin valve MR head consisting of a multilayered structure formed on a substrate. Both the aforementioned patents describe a spin valve MR head having a first and second thin film layer of magnetic material separated by a thin film layer of non-magnetic material. The magnetization direction of the first ferromagnetic layer in the absence of an applied magnetic field must be substantially perpendicular to the magnetization direction of the second ferromagnetic layer which is fixed in position.
U.S. Pat. No. 5,287,238 to Baumgart et al., describes a variation on the structure of the spin valve MR head disclosed in the Dieny patents. Baumgart et al. discloses a dual spin valve MR head having first, second and third ferromagnetic layers separated from each other by nonmagnetic layers. The outer ferromagnetic layers in the structure have their magnetic orientation fixed while the middle layer is comprised of soft ferromagnetic material which is free to rotate in magnetic direction in cooperation with the field from the media. As in the Dieny patents, for the described spin valve MR head to work, the magnetic direction of the middle rotating sensor element layer in Baumgart et al. is oriented perpendicular to the magnetization direction of the fixed outer layers when the applied field is zero.
In these known spin valve MR heads which have only one stable magnetization state, an analog output signal is detected by applying a small sense current to the head structure. The voltage output is an analog signal in that it continuously varies as a function of the MR head's resistance. The maximum change in magnetic orientation of the sensor element layer is limited to 90 degrees from its easy axis and typically must be constrained to even more limited rotation to provide for operation in the linear range of the MR material's transfer curve. Therefore, as such a known spin valve MR head passes over a magnetic transition on the recording media the magnetization direction of the sensor element is rotated a maximum of 90 degrees in a time varying manner, causing a change in resistance in the material and a resultant analog voltage output waveform.
In order to recover the actual recorded user data this analog output signal is typically converted into a digital signal during the demodulation process, which typically involves relatively complex peak detection circuitry. In recording channels using partial response maximum likelihood ("PRML") detection, a Viterbi algorithm is utilized that is even more complex than traditional peak detection. Additionally, since in digital recording systems the user data to be stored on the magnetic media is typically written in non-return to zero ("NRZ") format, the non-return to zero inverted (NRZI) data output of spin valve MR heads must then be converted back to NRZ format to recover the user data.
Accordingly, it would be desirable to provide an MR head capable of producing a digital output in order to simplify magnetic flux transition detection schemes. Also, it would be desirable to eliminate the need for transformation of the recovered data from NRZI to NRZ format.